While the world waits to see whether the United States will respond with military force to the suspected use of chemical weapons by the Syrian government against its own citizens, defense officials have likely been formulating attack plans ever since the president declared the use of such weapons a “red line,” experts said.

“They probably had some strike options in planning for quite some time,” said Ret. Col. Peter Mansoor, a professor of military history at Ohio State University who was Gen. David Petraeus’ executive officer in Iraq. “These plans have been batted back and forth at this point for some time now.”

President Barack Obama held a National Security Council meeting at the White House on Friday morning regarding Syria, officials told NBC News.

Later, the administration made a forceful case for limited U.S. military action, releasing evidence the Syrian government had used chemical weapons against civilians multiple times in the past year and saying the "indiscriminate, inconceivable horror'' could not go unpunished.

“I think a lot of the conversations are finished and they’re basically waiting for the execute order here from the president,” said Jeffrey White, a defense fellow at The Washington Institute for Near East Policy, a D.C.-based think tank.

“I would think that they would have narrowed the options way down,” White said. “I can’t say whether there’s just one he’s going to decide on. He may well have a couple, two or three, he could decide on.”

The White House has so far not received the widespread political and popular support it would most likely want before engaging in military action, with top administration officials including Secretary of State John Kerry working to gather congressional backing on Thursday. The British Parliament voted down a proposal for a military strike against Syria, and an NBC News poll released on Friday morning showed that half the American population said the U.S. should not intervene.

“Some cite the risk of doing things,” Secretary of State John Kerry said in prepared remarks Friday, as a U.S. government preliminary assessment determined that 1,429 people were killed in the Aug. 21 attack. “We have to ask what is the risk of doing nothing.”

Four U.S. Navy destroyers waited in the Mediterranean on Friday, armed with Tomahawk cruise missiles that are each capable of carrying a 1,000-pound bomb. Yet even the super-accurate Tomahawks cannot take out dangerous chemical weapons facilities directly, said Aaron David Miller, a distinguished scholar at the Wilson Center. That means even the most narrowly targeted strike could likely include direct hits on the Syrian military arms that carry them.

US guided-missile destroyer USS Preble conducting an operational tomahawk missile launch while underway in a training area off the coast of California, USA.Mc1 Woody Shag Paschall / Handou / EPA

“Syria is the single largest repository of this stuff [chemical weapons], so as a consequence you go after the delivery systems, fuel depots, airways, command and control,” Miller said. “And then you broaden it out from there to go to artillery units that have lost these things, aircraft that have dropped them.”

The U.S. military has weapons other than Tomahawk missiles that could be used to target chemical weapons facilities with the objective of destroying the stockpiles, Mansoor said.

In a July letter to the Senate Committee on Armed Services, Gen. Martin Dempsey said that American military forces were prepared for a number of options, including training the Syrian opposition, establishing a no-fly zone, and conducting strikes “to prevent the use or proliferation of chemical weapons.”

“We do this by destroying portions of Syria’s massive stockpile, interdicting its movement and delivery, or by seizing and securing program components,” Dempsey wrote in July, saying that seizing control of the majority of the regime’s chemical weapon supply would involve ground troops. “At a minimum, this option would call for a no-fly zone as well as air and missile strikes involving hundreds of aircraft, ships, submarines, and other enablers.”

In the event of a strike of any size, it may take days or longer to determine the effect on Assad's power.

“How do you measure the impact we had on the Syrian regime’s thinking about the situation? That’s tricky, and can only be measured if we see a response and a change in behavior,” White said.

At best, a strike could show that the U.S. is serious about the deteriorating situation in Syria, Mansoor said, while at worst it could convince Assad that Obama has played his one hand. After limited, separate cruise missile strikes against al-Qaeda forces in Afghanistan and Iraqi dictator Saddam Hussein in the late 1990s, the targets were only "emboldened," he said, as it seemed the U.S. was not "willing to do something even more."

And while the administration may have narrowed down its strike plans, decisions can be made right up to the time cruise missiles are fired or bombs are dropped, White said.

“I remember a case in a previous administration, I won’t get into the details, where the president was presented with targets to strike and he looked at the images and he said, ‘What’s this over here?’ and he was told what it was,” White said. “And he said, ‘OK, why don’t we hit that.’”